Merge pull request #170 from juukie/juukie-typo-world

Update message to match example

Author: yyx990803

src/guide/custom-directive.md

@@ -118,7 +118,7 @@ Vue.directive('demo', {
 var demo = new Vue({
   el: '#demo',
   data: {
-    msg: 'world!'
+    msg: 'hello!'
   }
 })
 </script>